The specific aims of the project are: 1) to examine the relative influence of four aspects of the adolescent's social network on cigarette, marijuana, and alcohol use, and to determine whether there are differences as a function of grade, sex, ethnicity, or type of substance, 2) to determine what variables impact friendship choice in adolescents, and to determine whether there are differences as function of grade, gender, or ethnicity, 3) to develop a model of substance use incorporating both selective association and peer influence.